To wank, or be wanked, in a particularly aggressive or over-enthusiastic manner. Usually involves an element of discomfort on the part of the person whose penis is being so manipulated.
Origins disputed, though commonly thought to stem from the mastubatory practices of Slovakian men, who, too used to driving tractors with rusty or stiff gear sticks, would manipulate their penises in a similarly vigorous fashion.
The style then spread to the gay community, where the virile masculinity of the Slovakian form worker was fetishised by the new intelligentsia and middle-classes. They therefore adopted the wanking style as a way of being more "masculine".
Nowadays if you are being wanked in such a fashion it is unlikely that the man wanking you is a genuine Slovakian tractor driver.
P: Bloody hell - he was pulling on my dick so hard it almost came off!
R: Oh - do you not enjoy a Slovakian Power Wank?